Page 140 - Handbook of Energy Engineering Calculations

Amidst the many advantages of gas turbine (GT) combined cycles (CC)

popular today from various standpoints (lower investment than for new
greenfield plants, reduced environmental impact, and faster installation and
startup), one drawback is that the achievable output decreases significantly as
the ambient inlet-air temperature increases. The lower density of warm air

reduces mass flow through the GT. And, unfortunately, hot weather typically
corresponds to peak power loads in many areas. So the need to meet peak-
load and power-sales contract requirements causes many power engineers
and developers to compensate for ambient-temperature-output loss.

The three most common methods of increasing output include: (1)
injecting water or steam into the GT, (2) precooling GT inlet air, and/or (3)
supplementary firing of the heat-recovery steam generator (HRSG). All three
options require significant capital outlays and affect other performance

parameters. Further, the options may uniquely impact the operation and/or
selection of other components, including boiler feedwater and condensate
pumps, valves, steam turbine/generators, condensers, cooling towers, and
emissions-control systems.

2. Evaluate and analyze inlet-air precooling
Evaporative cooling, Case 1, Table 1, boosts GT output by increasing the
density and mass flow of the air entering the unit. Water sprayed into the
inlet-air stream cools the air to a point near the ambient wet-bulb
temperature. At reference conditions of 95°F (35°C) DB and 60 percent RH,

an 85 percent effective evaporative cooler can alter the inlet-air temperature
and moisture content to 85°F (29°C) and 92 percent RH, respectively, using
conventional humidity chart calculations, page 13.104. This boosts the output